And I have solved the problem. Here's the clue for future viewers:
1. Using USB connection moved the webbrowser.app file from /applications to root folder
2. Started the app from root folder
3. Switched off / on the reader
4. Started the app again from root folder
(Actually I'm not sure whether 2. or 4. or both were necessary)
5. Using application explorer in PB UI moved the webbrowser.app back to /applications folder
Best regards.